A lithium-ion battery used in electric cars or the like is constituted by a positive electrode active material provided around a positive electrode current collector and a negative electrode active material provided around a negative electrode current collector with a separator being interposed between both active materials. Aluminum is often used for the positive electrode current collector and copper is often used for the negative electrode current collector. Lithium-containing manganese oxide or the like is used for the positive electrode active material and graphite or the like is used for the negative electrode active material. The separator is an organic substance in the form of a thin film with micropores, and an electrolyte is separated into a solute and a solvent.
In such a conventional lithium-ion battery, a battery cell and a cell monitoring system for detecting temperatures, voltages, and so on of the cell are provided as separate components and no function for heat dissipation is provided inside the battery cell; therefore, a cell monitoring system and a heat dissipation function are provided outside the battery cell to form a module and a plurality of such modules are provided to form a battery pack. When modules and a battery pack are formed in such a way, the energy density is reduced to half or less in comparison with the energy density of a single battery cell.
One of the reasons for which it is impossible to attain high energy density, as described above, is related to safety issues. Among reasons for decrease in safety, the first reason is electrolyte liquid leakage of the separator. When there is liquid leakage, lithium hexafluorophosphate as a solute dissolves in water to emit hydrofluoric acid, which is hazardous because of being toxic to the human organism and also flammable. The second reason is the possibility of overcharge. In the state of overcharge, a positive electrode material emits oxygen (O), which may result in catching fire. The third reason is the possibility of short circuiting. Occurrence of short circuiting may lead to sudden temperature increase inside the cell, which may result in catching fire. Short circuiting includes internal short circuiting and external short circuiting. Internal short circuiting is short circuiting that is caused by metallic contaminants mixing in during manufacture, but today, factories are under good control, so internal short circuiting does not occur. Meanwhile, external short circuiting may still occur.
Regarding these safety issues, no sufficient measures have been taken so far against liquid leakage, overcharge, or high temperatures in battery cells. Further, as a result of placing importance on safety, attempts are made to obtain margin for safety in the process of cell manufacture, module manufacture, battery pack manufacture, or installment in electric cars, which essentially have little relevance to safety itself but result in the battery system becoming heavier than necessary and also lead to a decrease in capacity for practical use.
Patent Document 1 provides measures against liquid leakage and overcharge among the issues described above. In the invention of Patent Document 1, for the purpose of reducing a volume of an overall size of a cell monitoring system and a cell used for a non-aqueous electrolyte secondary battery, a battery device is developed in which an electric circuit substrate on which the cell monitoring system is arranged and a non-aqueous electrolyte battery are mounted in a single housing and electro-conductive portions of the electric circuit substrate and the electric circuit element are covered with an insulator that is resistant to a non-aqueous electrolyte solution.
Patent Document 2 relates to a non-aqueous electrolyte secondary battery, in which, for the purpose of ensuring safety by reliably detecting an increase in battery temperature without compromising on power capacity and properties even for medium- or large-capacity batteries, a heat-sensitive element is disposed in a columnar space formed in the central part of a cylindrical electrode group that is wound in the shape of a spiral, i.e., the heat-sensitive element is disposed in an inner-diameter space of the electrode group.
Patent Document 3 aims to provide a laminate-type battery that is provided with a laminate electrode body with a large number of layers and a wound electrode body with a large number of winds, is highly reliable, and has a high degree of freedom in the position at which an external terminal is drawn out. Here, provided is a battery in which an electrode body including a sheet-like positive electrode, sheet-like negative electrode, and separator is accommodated in a laminate film exterior body, where the battery includes a laminate electrode body in which the number of electrode layers is 20 or more or a wound electrode body in which the number of winds is ten or more; a positive electrode tab laminate, in which a plurality of positive electrode tabs relating to the sheet-like positive electrode are laminated, is divided into a plurality of positive electrode tab laminates that are each connected to different positive electrode external terminals, and the positive electrode external terminals are electrically connected to each other either inside or outside the laminate film exterior body; and a negative electrode tab laminate, in which a plurality of negative electrode tabs relating to the sheet-like negative electrode are laminated, is divided into a plurality of negative electrode tab laminates that are each connected to different negative electrode external terminals, and the negative electrode external terminals are electrically connected to each other either inside or outside the laminate film exterior body.